The quantity of CO2 emitted by motor vehicles can be reduced in particular by lightening said motor vehicles. This lightening may be achieved by:
an increase in the mechanical properties of the steels constituting the structural parts or skin parts; or
a reduction in the density of the steels for given mechanical properties.
The first approach has been the subject of extensive research, steels having been proposed by the steel industry that have a strength ranging from 800 MPa to more than 1000 MPa. The density of these steels however remains close to 7.8, which is the density of conventional steels.
A second approach involves the addition of elements capable of reducing the density of the steels. Patent EP 1 485 511 thus discloses steels having additions of silicon (2-10%) and aluminum (1-10%), with a ferritic microstructure, and also containing carbide phases.
However, the relatively high silicon content of these steels may in certain cases pose coatability and ductility problems.
Also known are steels containing an addition of about 8% aluminum. However, difficulties may be encountered when manufacturing these steels, in particular during cold rolling. Roping problems may also be encountered when drawing these steels. When such steels contain more than 0.010% C, the precipitation of carbide phases may increase brittleness. The use of such steels for manufacturing structural parts is then impossible.